UI plugins custom features (#8137)

* initial implementation to let plugins provide custom ui features

* provide exportable types

* refactor ref into renderContext to make it more generic and support template preview area ui plugins

* rename 'renderContext' -> 'featureContext' as not all features may render something

* allow to specify the function name via the source file string divided by a colon

* Bump api version

* add tests

* add docs

* add docs

* debug: workflow

* debug: workflow

* fix tests

* fix tests hopefully

* apply suggestions from codereview

* trigger: ci

* Prove that coverage does not work

* Revert "Prove that coverage does not work"

This reverts commit 920c58ea6f.

* potentially fix test???

src/backend/InvenTree/plugin/samples/integration/user_interface_sample.py

@@ -122,3 +122,36 @@ class SampleUserInterfacePlugin(SettingsMixin, UserInterfaceMixin, InvenTreePlug
             })
 
         return panels
+
+    def get_ui_features(self, feature_type, context, request):
+        """Return a list of custom features to be injected into the UI."""
+        if (
+            feature_type == 'template_editor'
+            and context.get('template_type') == 'labeltemplate'
+        ):
+            return [
+                {
+                    'feature_type': 'template_editor',
+                    'options': {
+                        'key': 'sample-template-editor',
+                        'title': 'Sample Template Editor',
+                        'icon': 'keywords',
+                    },
+                    'source': '/static/plugin/sample_template.js:getTemplateEditor',
+                }
+            ]
+
+        if feature_type == 'template_preview':
+            return [
+                {
+                    'feature_type': 'template_preview',
+                    'options': {
+                        'key': 'sample-template-preview',
+                        'title': 'Sample Template Preview',
+                        'icon': 'category',
+                    },
+                    'source': '/static/plugin/sample_template.js:getTemplatePreview',
+                }
+            ]
+
+        return []